Instrumentation, Controls, and Automatic Operation
Core concepts
Instruments turn process variables into signals: pressure transmitters, RTDs and thermocouples, orifice and venturi flow elements, level devices (differential pressure, conductivity, float, radar), and UV or IR flame scanners. Distinguish indicating, alarm, and trip setpoints, and remember that trips must fail safe - fuel valves close on loss of signal or power.
Burner management and combustion control follow a fixed sequence: start-up purge with air flow proven, pilot light-off, main flame establishment, and flame-failure response that cuts fuel quickly. Interlocks typically include low-water cutoff, high steam pressure, low fuel pressure, low atomizing medium, and loss of flame or combustion air.
Drum level control: single-element (level only) suits steady small boilers; two-element adds steam flow; three-element uses steam flow, feedwater flow, and drum level, acting essentially as feedforward with level trim so it anticipates load swings.
Controllers may be on-off, modulating, or PID. Proportional band, integral, and derivative settings govern stability.
Traps
Expect questions labeling three-element control as level feedback only, or treating an alarm as an automatic trip. Also watch for loss of flame answered with increase fuel - always wrong.
Worked example
A level transmitter calibrated 0 to 100 in. of water outputs 4 to 20 mA. At 60 in., signal = 4 + (60/100) x 16 = 13.6 mA.